xz-utils is now out for quite a while and some important projects use it, e.g. the new version of coreutils, see http://article.gmane.org/gmane.comp.gnu.core-utils.announce/51 (The current workaround to use a three times longer additionally provided tarball is more a hack than an appropriate solution; moreover, who knows for how long such a longer tarball will be provided by upstream). Also EAPI=3 should have support for such .xz tarballs. Actually, as the current eix maintainer, I am also thinking about switching the tarball format to (only) xz. However, xz-utils still cannot be installed on a typical gentoo system, because it blocks lzma-utils which is needed by really many packages. This block is appropriate, because it replaces lzma-utils. However, it replaces it in a downward-compatible manner, i.e. all packages depending on lzma-utils could instead depend on xz-utils because the lzma/unlzma binary from xz-utils does the same as that of lzma-utils and has the same syntax: I did emerge -e @system @world only with xz-utils without any problems, i.e. I can confirm that no changes expect for the dependencies are necessary. Therefore, I suggest to introduce virtual/unlzma (which was also suggested in bug 218459) and|or virtual/lzma and to change all the dependencies on app-arch/lzma-utils to this virtual package.
The main problem I see here is that the final version is yet to be released. Though this does seem like a good idea.
*** This bug has been marked as a duplicate of bug 262008 ***
